As DimkaM said, you just switch into TV mode and then there is no scanline doubling.However, horizontal sync changes to 15kHz and it is very rare for any VGA monitor to support TV mode in their VGA ports.

Is is in future open possibility to add it as option or is it hardware limited ?I need implemented Scanlines to VGA, because my LCD TV kills scanlines from RGB input too :-(Nice solution will have Spectrum Next - is possible to switch Scanlines ON/OFF in realtime: